1. Field of the Invention
The present invention relates to a sheet processing apparatus that processes stacked sheets.
2. Description of the Related Art
A sheet processing apparatus, which bundles sheets on which images are formed by an image forming apparatus (a printer) and then makes the sheets into a book form by binding and folding the sheets, is known. For example, there is a sheet processing apparatus that sequentially receives sheets by a tray, aligns the sheets into a bundle, binds the sheet bundle at a position near the center, pushes the sheet bundle into a nip of a folding roller pair by poking with a pushing member, and folds the sheet bundle by the folding roller pair during conveyance.
In such a process that makes sheets into a book form by binding and folding a sheet bundle, it is necessary to align the sheets in a longitudinal direction (a sheet conveyance direction) and a width direction (a direction parallel to a sheet surface and perpendicular to the sheet conveyance direction) when bundling the sheets. For example, a conventional method stops the front ends of sheets conveyed by a sheet positioning member, and aligns the sheets in the sheet longitudinal direction according to the inertia force of the sheets for aligning the sheets in the sheet longitudinal direction (see Japanese Laid-Open Patent Publication (Kokai) No. 2009-126592 (JP 2009-126592A)).
However, the above-mentioned sheet alignment method may not be able to align shin sheets with small basis weight in the sheet longitudinal direction correctly because of the small inertia force. Since coated paper sheets with smooth surface quality tend to stick each other, the sheets are not correctly adjusted in the sheet longitudinal direction when the inertia force of the sheets does not exceed the force of sticking sheets.